A method of sending data by using a random access is used as one of conventional wireless communication methods. For example, in a mobile unit communication system, when uplink data is generated, a mobile station performs a random access procedure by using a physical random access channel (PRACH) and requests uplink transmission grant from a base station. With random access, contention occurs if multiple mobile stations send the same shared preamble ID on the same PRACH. If contention occurs, the base station is not able to detect a mobile station that has performed the random access. Hereinafter, such random access will be referred to as contention-based random access.
In the following, the conventional operation of the contention-based random access will be briefly described. For example, if uplink data is generated, the mobile station notifies, by using a PRACH, the base station of an Msg1 that contains a randomly selected shared preamble ID. At this point, if multiple mobile stations send the same shared preamble ID by using the same PRACH, contention occurs. Then, by using an Msg2, the base station sends back a response to the Msg1 together with a synchronization signal, transmission grant, and the like that are used for uplink communication. Then, the mobile station acquires the uplink synchronization timing from the Msg2. The mobile station then sends its own identifier and the like by using an Msg3. If the base station detects the identifier of the mobile station, the base station sends an Msg4 to that mobile station, thus resolving the contention. Then, the mobile station starts to exchange data with the base station after exchanging the Msg1 to Msg4. As for the examples of the conventional technologies, see 3GPP TS36.300, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)”, V10.0.0, Release 10, June 2010, and 3GPP TS36.321, “Medium Access Control (MAC) protocol specification”, V9.3.0, Release 9, June 2010, for example.
However, with the contention-based random access described above, the time period (Msg1 to Msg4) taken to perform synchronization is about 25 ms, which is a problem in that it is a significant loss of time.